Three o&#39;clock welding method in narrow groove

ABSTRACT

A three o&#39;clock welding method in a narrow groove, wherein a lateral butt welded joint is formed by carrying out welding in a narrow groove. The narrow groove is inclined with respect to the horizontal in such a manner that the center face of substantially parallel upper and lower groove faces is lowered on the initial layer side, when the welding is performed.

This invention relates to a three o'clock welding method in a narrowgroove in which welding wire material formed in wave shape is fed to acontact tube for carrying out three o'clock welding.

An apparatus for carrying out three o'clock welding in a narrow groovedisclosed in U.S. Pat. No. 4,188,526 has largely been in use forperforming downhand welding, such as stub welding, in thick plates and aheader. The welding carried out by using this apparatus best suits thejoining by welding of thick plates of narrow groove and can offer theadvantages that the deposited metal is small in amount and an sound weldcan be obtained by stacking layers of single beads. Thus, the apparatusin conducive to improved quality of the weld, economizing on expenses,shortening the period of time for carrying out work, and the need toexpand the scope of applications of the apparatus has been felt.

An object of this invention is to provide a welding method capable ofcarrying out three o'clock welding by using the aforesaid apparatus.

Another object is to provide a method of three o'clock welding such forcarrying out a welding of an upwardly advancing and/or downwardlyadvancing narrow groove.

A further object is to provide a method of joining a branch pipe to abody or pipes by three o'clock welding in a narrow groove.

The three o'clock welding method in a narrow groove according to theinvention is characterized in that the groove includes substantiallyparallel upper and lower groove faces in which the center face thereofis inclined in such a manner that the initial layer side of lowered withrespect to the horizontal.

The method according to the invention for welding an upwardly advancingand/or downwardly advancing narrow groove by three o'clock welding in anarrow groove is characterized in that the amount of ejected shield gassupplied to nozzles located posteriorly and anteriorly of the directionin which welding by a welding torch progresses can be controlled inaccordance with the welding position.

The method according to the invention for joining by welding a branchpipe to a base pipe in three o'clock welding is characterized in that anI narrow groove is provided in which the generating lines of an invertedtrapezoidal cone surface formed by the center line of the groove crossthe center axis at a constant angle at all times and that welding iscarried out by GMA welding means.

Other objects, features and advantages of the invention will becomeapparent from the description of the embodiments set forth hereinafterwhen considered in conjunction with the accompanying drawings.

FIG. 1 is a fragmentary sectional view of the narrow groove weldingmeans including a contact tube shown in U.S. Pat. No. 4,188,526;

FIG. 2 is a perspective view of the narrow groove welding apparatus;

FIG. 3 is a side view of the welding torch;

FIG. 4 is a vertical sectional view of the welding torch;

FIG. 5 is a sectional view taken along the line V--V in FIG. 4;

FIG. 6 is a sectional view showing the formation of beads when thecenter face between the groove faces is horizontal;

FIG. 7 is a sectional view showing the groove shape according to theinvention and the manner in which bead formation takes place;

FIG. 8 is a view showing the relation between the gap G and the angle ofinclination θ of the center face and a region suitable for working theinvention;

FIG. 9 is a sectional view of a test piece used for setting the angle ofinclination;

FIG. 10 is a perspective view of a model of upwardly advancing welding;

FIG. 11 is a perspective view of a model of downwardly advancingwelding;

FIG. 12 is a view in explanation of the apparatus according to theinvention;

FIG. 13 is a fragmentary sectional view of the vicinity of the gasejection parts or nozzles;

FIG. 14 is a fragmentary sectional view of the welding torch in whichthe water cooling tube is in the form of a double tube;

FIG. 15 is a sectional side view of the welding torch shown in FIG. 14;

FIG. 16A is a sectional view of a base tube and a branch tube showing awelding method of the prior art;

FIG. 16B is a view of a base tube and a branch tube showing the weldingmethod according to the invention;

FIG. 17 is a perspective view of the apparatus for carrying outautomatic welding;

FIG. 18 is a view of a model showing the movement of a welding torch ofthe prior art;

FIG. 19 is an exploded perspective view of the welding apparatusaccording to the invention;

FIG. 20 is a sectional view taken along the line XX--XX in FIG. 19;

FIG. 21 is a perspective view showing a branch tube joined to a basetube;

FIG. 22 is a view of a model of the groove shown in FIG. 21;

FIG. 23 is a perspective view of a branch tube welded to a base tube;

FIG. 24 is a sectional view of a branch tube connection using a weldingmeans of the prior art in which the right half portion is the X-Ysection of FIG. 23 and the left half portion is the Y-Z section of FIG.23;

FIG. 25 is a vertical sectional view of a weld between a branch tube anda base tube of large thickness;

FIG. 26 is a sectional view corresponding to FIG. 24 and includingnarrow groove according to the working of the invention;

FIG. 27 is a perspective view of a saddle-like curve of the end face ofa branch tube; and

FIG. 28 is a perspective view of the gas metal arc (GMA) branch tubeautomatic welding device used in working the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a welding wire (solid wire) 1 of a small diameteris formed into a wave shape and supplied to a weld zone through acontact tube 5. The wire extending from the forward end of the apparatusis swung rightwardly and leftwardly in FIG. 1 as the welding progresses,to automatically move the arc in swinging movement to give completepenetration to the side wall surface of the narrow groove formed betweenopposed surfaces of metal members 6 to enable a defectless sound weld orthe metal members 6 at a backing material 3.

The welding wire 1 is wound on a wire reel, not shown, and led to wirefeed rollers 4 through a swing plate 2, and reaches the weld zonethrough the contact tube 5 while having a current passed therethrough.

In this process, the welding wire 1 is subjected to plastic deformationby the action of the swinging plate 2 to be formed into a wave shape byusing the outer periphery of each feed rollers 4 as a bending guide.When the welding wire 1 reaches the welding section, the welding wire 1is restored to its original shape to move the arc in swinging movement.

In FIG. 2 the The mechanism for bending the welding wire 1 is distinctfrom the mechanism shown in FIG. 1 and bending is effected by a swingingmovement of the bending rollers 14. The swinging movement of the bendingrollers 14 is caused by an eccentric drive 13 connected with bendingrollers 14, with the wire 1 being fed to the welding torch 12 by wirefeed rollers 11 driven by a drive motor 10. The wire 1 is fed from afeed roller 7 by a drive means 9 through a guide tube 8 to bendingrollers 14. However, the two mechanism have the same object of formingthe welding wire 1 into a wave shape.

As shown in FIGS. 3, 4 and 5, the welding torch 12 is composed of thecontact tube 5, colling water feed pipes 16, 17, a cooling water returnpipes 16', 18, an upper block 15 and a contact tip 19.

When the welding torch 12 of this construction is used for carrying outwelding in one direction only, the welding torch is usually used bykeeping the flow rate of gas constant in the forward and backwarddirection.

When the side surface of the groove face is horizontal as three o'clockwelding is carried out by using the narrow groove welding apparatus ofthe aforesaid construction, bead formation will be as shown in FIG. 6 inwhich as welding progresses from the initial layer bead in contact witha backing strip 21 and the surfaces 20a of the beads 20 gradually showtilting as the number of layers increases, due to the balancing betweenthe force of gravity to which molten metal is subjected and surfacetension, the inert gas velocity released from the nozzle and the coolingspeed, until finally a narrow groove welding is made impossible to carryout by the stacked layers of single pass as shown.

In view of the foregoing, an imaginary center face was set substantiallymidway between opposite sides of the groove and experiments andmeasurements were conducted on the inclination of the center face andformation of the surfaces of the beads. 6 mm was selected as a minimumdimension of a gap in view of the minimum essential width (that enablesthe contact tube to enter the gap of the groove) of the contact tube ofthe apparatus in relation to the swinging of the end portion of thewelding wire or the arc generating point. Meanwhile, 12 mm was selectedas a maximum dimension of the gap in view of the need to minimizedeposited metal and increase the effects achieved in saving time andmaterial by carrying out narrow groove welding by a single pass. Withregard to the angle of inclination of the bead surface 20a, itascertained that as shown in FIG. 7 an optimum zone exists with respectto the gap G for the angle of inclination θ formed by a center face 22and the horizontal for carrying out narrow groove welding by a singlepass, in view of the inert gas ejection velocity from the nozzle, therelation between the temperature for producing molten metal and surfacetemperature and cooling velocity (by the base metal). It was alsoascertained through experiments that this zone is a hatched zone shownin FIG. 8 which is bounded by A and B curves (the a curve indicating anupper limit and the B curve a lower limit).

The lower the surface tension of the molten metal, the greater the angleof inclination. The provision of an inclination of over 45° isundesirable because the amount of deposited metal increases in the samefashion as when the plates to be welded have their thickness increased.Even if the groove is as shown in FIG. 6, when the members 6 to bewelded are such that the base material can be tilted, it is possible tocarry out welding by tilting the base material to thereby tilt thecenter face of the groove.

In respect of the angle of inclination θ, the angle of inclination θ canbe decided by setting a test piece to a gap G corresponding to the gapto be joined and forming beads with three to four passes by placing thecenter face of the groove horizontal, so that the angle α formed by thetangent (shown in FIG. 9) of a curve indicating the bead surface at apoint (with respect to a cross section) Y at which the weld bead surfaceof the final bead (the fourth bead, for example) and the center face ofthe groove intersect each other can be checked by the cut surface of thetest piece. In this case, it is essential that attention be paid to thefact that the velocity at which inert gas is ejected from the nozzle isan important factor is deciding the angle of inclination θ.

The conditions under which the aforesaid welding method was performedwhere as follows

Base material: SA 299 (ASME) SB 49 (JIS)

Current: 220a Voltage: 26 V,

120 Hz pulse: Welding wire diameter: 1.2 mm

Wire material: MGS 50 (carbon steel wire)

Inert gas: argon gas containing 20% CO₂ fed at 25 l/min

Welding velocity: 250 mm/min

Range suitable for practical use θ is below 45 degrees

Groove gap dimension: 8-12 mm.

Under these conditions, the hatched zone shown in FIG. 8 was found to beoptimum for effecting welding in relation to the angle of inclination θand the groove gap (G) dimension.

Generally, it is necessary that the angle of inclination θ be set at alarge value when the molten metal tends to flow and the heat istransmitted poorly to the base material. Thus, it is necessary that thevelocity at which the inert gas is ejected from the nozzle be slightlyincreased.

By working this invention, three o'clock welding in a narrow groove canbe readily carried out merely by rendering the groove a simple I squarebutt joint or substantially parallel sided narrow groove and setting theangle of inclination in accordance with the gap size. In inert gas metalarc welding, the invention enables the effects of holding the inert gasto be readily achieved and allows inert gas atmosphere to be readilyformed, so that automatic welding can be readily performed by supplyingthe welding wire smoothly. Thus, the invention can achieve the resultsof improving the quality of the weld and economizing on expenses.

An embodiment of a method of and an apparatus for carrying out threeo'clock welding in an upwardly advancing and/or downardly advancingnarrow groove is shown in FIG. 10, wherein a backing material 141' isjoined to the members by beads 143a inclined at their surface by anangle α. FIGS. 11, 12 show the apparatus suitable for working theinvention which is shown as carrying out downwardly advancing welding.In this case, a bead 143d is inclined at its surface at an angle α withrespect to the horizontal l and the backing strip 141' is inclined bythe angle of inclination β in that portion while the center axis of thecontact tube 105 is inclined by γ with respect to the horizontal. Thus,γ is associated with β and the condition for preventing the downflow ofthe molten metal for forming the beads or the condition for pushing upthe bead face is mainly determined by the angle γ. This can be decidedby the result of a test using a test piece shown in FIG. 9 in which thecenter face 22 is welded by causing same to tilt not only with respectto the horizontal but also at various angles and by the result of a testin which the angle β is varied with respect to the test piece.

In avoiding the downflow of the molten metal, the amount of the ejectedgas can be controlled by increasing the amounts of gas ejected throughan anterior gas ejection port (hereinafter referred to as a nozzle) 125and a posterior gas ejection port or nozzle 126 or the amount of gasejected through the nozzle 125. Control of the amount of the ejected gascan be effected by adjusting shield gas flow rate control valves 144aand 144b. The contact tube 105 has mounted thereon a tube tilting anglegenerator 145 generating tube tilting signals which are transmitted to acontrol box 147 via an angle indicator 146. Information on the amountsof the inert gas ejected through the nozzles 125 and 126 with respect tothe angle of inclination γ of the contact tube 105 obtained by theexperiments is stored in the control box 147, so that it is possible toadjust the shield gas flow rate control valves 144a and 144b in such amanner that the inert gas can be ejected through the nozzles 125 and 126in amounts optimum for the angle of inclination γ. The tube tiltingangle generator 145 may be attached to an end (not shown) of an armconnected to the welding torch so that it will move along the outersurface of the members to be welded or along the leading bead face.

FIG. 13 shows the detailed construction of the contact tube 105. Optimumresults can be achieved in working the invention by forming the shieldgas ejection ports or nozzles 125 and 126 as having diversion typeopenings and by tilting the contact tube 105 in such a manner that in avertical section including the center axis of the contact tube,sectional walls 125a and 126a of all the walls of the opening which areremote from the center axis of the contact tube are parallel to thecenter axis and sectional walls 125b and 126b which are near to thecenter axis are inclined in a manner to have their end portions disposedclose to the center axis.

As shown in FIGS. 14 and 15, the cooling water tube is preferably formedin a double tube construction in which a cooling water feeding tube 117and a cooling water return tube 118 are formed as an inner tube and anouter tube respectively within the contact tube portion so that thecooling water will flow by changing its direction in the end portionnear the contact tip 119.

By working the invention, it is possible to achieve the effects ofwelding of a saddle type groove, such as a connection between a headerand a stub, which has an upwardly advancing portion and a downwardlyadvancing portion, to be carried out readily and automatically underoptimum conditions.

An embodiment of an apparatus for welding a narrow groove suitable forcarrying out three o'clock welding in which the center line of thegroove is a curve displaced in a horizontal direction and/or a verticaldirection as shown in FIG. 21 will now be described. A method of theprior art for welding a branch tube to a base tube shown in FIGS. 16Aand 16B will be outlined. As shown, the left half portion of the figurewith respect to the center line shows a circumferential section and theright half portion thereof shows a longitudinal section. In FIG. 16A, abranch tube 208 is placed on a base tube 207 and a deposited metal 215is provided to a groove formed on the branch tube 208, in carrying outwelding. Since the groove is a saddle type and has a complex shape,welding of the groove has been performed manually, as by gas cutting, inthe majority of the work done for preparing the edge. Thus, both edgepreparation and welding have hitherto required a lot of labor and a longperiod of time. In some applications, a submerged arc welding process ora short arc welding process has been adopted for carrying out welding bymechanically synchronizing vertical movement to rotation. FIG. 17provides and example of a welding apparatus for carrying out amechanical welding process in which a groove formed between the basetube 207 and the branch tube 208 is automatically welded by controllinga welding head 210 by the control box 212 via a saddle shape swivelingmechanism 209 and a side beam 214. 211 is a transformer for the weldingpower source and 213 a control panel. The movement of the welding headposes the problem that misoperation or trouble is liable to occur due tothe facts that the members to be welded have different sizes and thewelding conditions are complex. When the groove has a large width, theproblem arises that the welding bead sags and drops. In the presentinvention, lateral narrow grooves parallel in widthwise direction havebeen developed as shown in FIG. 16B in order mainly to avoid sagging ofthe welding bead and increase the strength of the weld. Morespecifically, a groove is formed in each of the base tube 207 and thebranch tube 208 to provide a groove 240 of constant width through theentire circumference. This facilitates automating of welding. Oneproblem is raised in this connection. Since the welding torch is fixedwith respect to the welding head, the welding torch 220 would becomeimpossible to move in X and Y sections when the groove 240 draws a curveas shown in FIG. 18. This has hitherto made it necessary to provide awide space for these sections of the groove, as indicated by the numeral241.

The invention has succeeded in eliminating the disadvantages of theprior art and making it possible to carry out welding economically withminimized expenses, to obtain a weld of high reliability by providing anessential minimum of the groove width, even if the center line of thegroove is a curve displaced in a horizontal direction and/or a verticaldirection.

Referring to FIG. 19, a boss 229 is formed in the welding torch 220 andhas a passageway 226 in the central portion for allowing the weldingwire to pass therethrough. A welding head 230 is provided with feedrollers 205 for feeding the welding wire. A swivel ring 223 is fittedover the boss 229 of the welding torch 220 and the welding head 230 sothat the welding head 230 supports the welding torch 220 in swivelingmovement. The welding torch 220 is formed at lower portion thereof witha substantially U-shaped profiling plate 227 located on the rear sidewall which is the trailing end with respect to the direction of movementof the welding torch 220. The profiling plate 227, which hassubstantially the same width as the groove, is adapted to come intocontact with the groove as the welding torch 220 moves within thegroove, so as to thereby correctly position the welding torch 220 withrespect to the groove at all times. The profiling plate 227 may bereplaced by guide plates 227a connected to the trailing side wall and/orthe leading side wall of the welding torch 220. The guide plates 227aare normally located in the leading end portion of the welding torch 220with respect to the direction of movement 222 thereof. The guide plates227a may be of the same shape as the profiling plate 227 or asubstantially of a pear shape, in which the open sides of thesubstantially U-shaped guide plate are bent inwardly. By using thisconstruction, contact of the welding torch 220 with the groove face asshown in FIG. 18 can be avoided. In FIG. 19, 224 is a shield gas supplytube, and 225 a cooling water tube. In FIG. 20, 224' is a shield gaspassageway, and 225' a cooling water passageway. FIG. 1 shows oneexample of the method for feeding a welding wire in the welding head 220shown in FIG. 19 which is based on a narrow groove welding methoddisclosed by the present inventor previously. This method which is avariation of the GMA welding method enables the swing plate 2 to move inswinging movement as indicated by arrows as the welding wire 1 passesthrough the swing plate 2 and is forced into the contact tube (weldingtorch) 5 by the feed rollers 4. This permits the feed rollers 4 to actas a bending die, to deform the welding wire 1 into a wave shape. As aresult, the welding wire 1, forced out of the contact tube 5, moves inswinging movement without having any directionality at the arcgenerating point, thereby enabling welding to be performed without anytrouble, such as a lack of penetration with respect to the side wallsurface of the groove.

FIG. 21 shows one example of a narrow groove in which the direction ofwelding is a curve displaced both in the horizontal direction and thevertical direction, for joining the branch tube 208 to the base tube 207by welding. The groove is a saddle type groove having a constant width,as shown in FIG. 16B. In this case, the width of the groove can bereduced to an essential minimum value (6-15 mm, for example) because theapparatus according to the invention is capable of moving freely withinthe groove. The saddle type groove shown in FIG. 16B is such that theangle at the center of the groove is constant with respect to the centeraxis 208a of the branch tube 208, so that the welding torch has only tomove vertically and in rotary movement. FIG. 22 shows in a model themanner in which the torch 220 moves within a groove 240 shown in FIG. 21with respect to the direction of movement of the torch 220. In FIG. 22,m, n and o indicate the points in the groove shown by the same numeralsin FIG. 21. In this case, the profiling plate 227 causes the weldingtorch 220 to be displaced for movement along the groove 240 whilekeeping the open sides of the U-shaped shape in contact with side walls240a and 240b respectively, of the groove space 240.

By working the invention, it is possible to freely move the weldingtorch in a lateral narrow groove of which the center line of the grooveforms an indefinite curve. Thus, it is possible to carry out welding ofthe lateral narrow groove of complex shape by means of an automaticwelding machine efficiently and positively.

FIG. 25 provides an example of a prior art welding method for welding abranch tube 302a, having a thickness T₂ and T₃, to a base tube 301ahaving a thickness T₁, with a backing strip 304a being welded to themembers. An electric heater 305a is provided for heating portions of theweld zone to prevent a cracking or other weld defects. The weld groovebetween the branch tube 302a and base tube 301a has a width W₁.

As shown in FIG. 23, in a boiler for a generating plant, pressure vesselof a nuclear reactor, pressure vessel of a chemical plant, etc., abranch tube 302 is connected to a base tube 301 in such a manner thatthe center axis of the former is at a right angle to that of the latterin many cases.

Generally butt welded joints are in many cases such that the groovefaces of the members to be butt welded have the same shape and thecenter line of one groove and the center line of the other groove are inone plane as in joining flat plates together by welding. In the buttwelded joint formed in connecting a branch tube to a base tube asdescribed hereinabove, the weld line is a curve of the saddle shapebecause the cylindrical branch tube 302 is attached to the outer surfaceof the base tube 301.

The shape of a groove to be welded (including a shape corresponding tothe numeral 303 and a gap O in FIG. 24) raises the problem that thecurve is so complicated in shape that it shows a stepless change insaddle shape from the top of the longitudinal extension of the base tube301 to the bottom thereof as seen peripherally and the center line ofthe groove does not exist in the same plane.

Because of this complicated shape, the groove has no definitemathematical and graphic principles and defies machining. Although gascutting operation may be adopted in some applications, working andfinishing must rely on manual operation after all in many applications.

In FIG. 26, an X-Y section (right half portion) is a longitudinalsection of the base tube and an X-Z section (left half portion) showsone-half the transverse surface of the base tube which is a peripheralsection thereof.

In accordance with the present invention a GMA automatic weldingapparatus is provided in which the welding wire 1 is fed to the contacttube (shown at 5 in FIG. 1) for welding thick plates with a narrowgroove of less than 10-8 mm. In using this machine, it is necessary thatthe groove face be formed on the basis of mathematically and graphicallyestablished principles. Also, the center axis of the contact tube 5 isrequired to change its position about the center axis at a constantangle β with respect to the center axis (Y-axis) of the branch tube. Itis inevitable that if the base tube of a larger diameter is joined tothe branch tube of a smaller diameter, the weld line be saddle shape.Forming of the weld line of the saddle shape will be describedgraphically by referring to FIG. 27. The numeral 306A indicates inphantom lines an inverted trapezoidal cone 306 having a bottom surfaceof the same outer diameter as the branch tube 302 and a vertical angleof 2β. In the inverted trapezoidal cone 306, the bottom face having thesame diameter as the inner diameter of the branch tube 302 is designatedby the numeral 306A'. FIG. 27 shows in a perspective view (in phantomdot-and-dash lines) the phantom inverted trapezoidal cone as it isforwardly inclined with respect to the perpendicular. The numeral 307designates one of the generating lines of the inverted trapezoidal cone.In this case, when the center of the bottom surface is moved in onereciprocatory movement the distance between position A and position B ora constant distance while the inverted trapezoidal cone (hereinafterreferred to as a cone) rotates through 180° with the axis O-Y as thecenter axis and it is moved in another reciprocatory movement betweenpositions A and B while the cone further rotates through 180 degrees, acurve m, n, o, p, m described by an end m of the generating line 307 isthe line of the saddle shape of the weld line 303. The relation betweenm and n or the dimension between A and B is decided by the diameter D₁of the base tube and the diameter D₂ of the branch tube. An end face308₁ of the branch tube moving in the saddle shape is formed by thesaddle-shape movement of the generating line 307 and its perspectiveview is shown by a curved surface drawn by a solid line. When a face308₂ having a width w₂ is formed on the base tube side corresponding tothe end face 308₁ of the branch tube, it is possible to form either bymachining or by means of a gas cutting torch a narrow groove of thewidth w₂. In FIG. 27 the depth of the weld groove is identical with thelength of the generating lines of the inverted trapezoidal cone whichlines are defined between the cone bottom face 306A having the samediameter as the outer diameter of the branch tube 302 and the bottomface 306A' having the same diameter as the inner diameter of the branchtube 302 with respect to the points (m) and (o). Similarly, the lengthof the generating lines between the bottom faces 306B and 306B' withrespect to the points (p) and (n) in the weld line is equal to the depthof the weld groove. Machining can be carried out with a high degree ofprecision to give the value of 10-8 mm to the width w₂, and the angle ofinclination β of the generating line 307 with respect to the center axiscan be made constant. Stated differently, the groove of the small widthof this type can be given with accurate shape and configuration, tofacilitate GMA automatic welding of the narrow groove.

By adopting this groove, it has become possible to use a backing strip309 of small thickness tube shape shown in FIG. 26 as a backing stripfor welding. As shown, an end portion 309₁ on the base tube side may notstick out of the inner surface of the base tube and may remain insidethe branch tube depending on the condition of the fluid. Alternatively,this portion can be removed by machining.

FIG. 28 is a perspective view of a GMA branch tube automatic weldingapparatus used for working the invention. Operation of the welding head316 requires the saddle shape drive of the welding head conforming tothe curvature of the surface of the base tube as described hereinabove.This has been achieved by driving a head rotating shaft 317 and a headvertically moving shaft 318 in accordance with the usual memoryfollow-up system by means of a microcomputer.

A head support body 319 moves on rails 320 and stops at an arbitrarilyselected position. The numeral 321 designates a rough adjustment beamfor setting the position of the welding head 316 by rough adjustments.

By mounting a gas cutting torch, not shown, on the outer casing of thewelding head 316 in place of the welding head 316, it is possible toreadily form a gas cutting groove of the saddle shape for both the basetube and the branch tube for keeping constant the angle of inclination βwith respect to the center axis, to enable edge preparation to becarried out with increased accuracy.

By virtue of the present invention, it is possible to form a groove ofthe same shape through the entire circumference by forming the groove onthe base tube side and inserting the branch tube (which is generallyreferred to as a set-in type mounting) while it has been the usualpractice to mount the branch tube by placing the same on the base tube(which is generally referred to as a set-on type mounting), therebyenabling edge preparation (gas cutting) and automation of welding to beachieved. Since the GMA welding means is able to carry out welding atlow thermal input (21 KJ/cm), it is possible to produce a weld of hightoughness having superior mechanical properties. In addition, thepaucity of diffusive hydrogen results in low hydrogen brittleness andimproved heat treatment condition. Owing to the facts that the narrowgroove welding method used uses a small amount of welding metal and thewelding method is automated, the working expenses are lower by about 70%than the corresponding amount required in a method of the prior art,including an edge preparation step.

What is claimed is:
 1. A three o'clock welding method in a narrow groovefor forming a lateral abutted joint, the method comprising the steps of:forming a groove which tilts such that the center face of upper andlower groove faces substantially parallel to each other is lowered withrespect to the horizontal on an initial layer side, and welding bydepositing molten metal in a narrow groove in layers, with each layerbeing formed by a single pass of a welding means, and wherein an angleof inclination of said center face with respect to the horizontal isselected in accordance with a gap dimension of the narrow groove.
 2. Athree o'clock welding method in a narrow groove as claimed in claim 1,further comprising the step of ejecting an inert gas through a nozzle ofthe welding means at a predetermined velocity for preventing a formedmolten bead from flowing downwardly when the bead is formed, and whereina thickness and heat conductivity of a base material, and a surfacetension of molten metal when a welding wire is melted to form the moltenmetal are used as factors for determining the angle of inclination ofthe center face.
 3. A three o'clock welding method in a narrow groovethe method comprising the steps of: forming a groove which tilts suchthat the center face of upper and lower groove faces substantiallyparallel to each other is lowered with respect to a horizontal on aninitial layer side, and welding by depositing molten metal in the narrowgroove in layers, with each layer being formed by a single pass of awelding means, an angle of inclination of said center face with respectto the horizontal is selected in accordance with a gap dimension of thenarrow groove, and wherein when a base material is steel, the gapdimension of the narrow groove is about 8-12 mm and the angle ofinclination of the center face of the groove with respect to thehorizontal is less than 45°.
 4. A three o'clock welding method in anarrow groove as claimed in claim 1, wherein the angle of inclination ofa tangent to a bead face at the center point of the bead face in a crosssection of a weld with respect to the horizontal is measured after testwelding is carried out by using a test piece of the angle of aninclination of zero, so as to select the angle of inclination.
 5. In athree o'clock welding method in a narrow groove for forming a transversebutt welded joint by an upwardly advancing and/or a downwardly advancingwelding, comprising the steps of:forming a groove which tilts such thatthe center face of upper and lower groove faces substantially parallelto each other is lowered with respect to the horizontal on the initiallayer side; and controlling the amount of shield gas ejected throughnozzles located anteriorly and posteriorly with respect to the directionof welding performed by means of a welding torch in accordance with theposition of welding so as to prevent a molten bead from flowingdownwardly toward the horizontal.
 6. A three o'clock welding method in anarrow groove for directly joining a branch tube to a base tube, themethod comprising the steps of:forming an end face of said branch tubeon a curved surface described by one generating line of an invertedtrapezoidal cone having an end face of a diameter equal to that of thebranch tube as said inverted trapezoidal cone moves axially at aconstant distance in two reciprocatory movements in one completerevolution about a center axis thereof; forming the narrow groovebetween said end face of the branch tube and a wall of the base tube,said narrow groove being inclined in such a manner that an initial weldlayer side is lower with respect to a horizontal on the initial layerside; and automatically welding the branch tube to the base tube so asto join the same to each other.
 7. A three o'clock welding method in anarrow groove as claimed in claim 6, further comprising the step ofsupplying a welding wire to a welding head while forming the weldingwire in wave shape, so as to join the branch tube to the base tube byGMA automatic welding.
 8. A three o'clock welding method in a narrowgroove as claimed in claim 6, further comprising the step of attaching abacking strip of the tubular shape in a branch tube when the automaticwelding is carried out.
 9. A three o'clock welding method in a narrowgroove as claimed in one of claims 6 or 7, further comprising the stepof arranging a backing strip of a tubular shape in such a manner that anend portion view of on the base tube side is prevented from sticking outof an inner surface of the base tube.
 10. A three o'clock welding methodin a narrow groove as claimed in one of claims 6 or 7, furthercomprising the step of moving the welding head in saddle shape movementnecessary for welding of the branch tube to the base tube in accordancewith instructions given by a microcomputer by a memory follow-up system.11. A three o'clock welding method in a narrow groove as claimed inclaim 10, wherein a gas cutting torch is mounted in place of a weldinghead or on the welding head for carrying out an edge preparation for thebase tube and the branch tube.
 12. A three o'clock welding method in anarrow groove for forming a lateral abutted joint, the method comprisingthe steps of forming a groove which tilts such that the center face ofupper and lower groove faces substantially parallel to each other islowered with respect to the horizontal on an initial layer side, weldingby depositing molten metal in the narrow groove in layers, with eachlayer being formed by a single pass of a welding means, ejecting aninert gas through a nozzle of the welding means at a predeterminedvelocity for preventing a preventing a formed molten bead from flowingdownwardly when the bead is formed, and wherein a thickness and heatconductivity of a base material, and a surface tension of the moltenmetal when a welding wire is melted to form the molten metal are used asfactors for determining the angle of inclination of the center face. 13.A three o'clock welding method in a narrow groove for forming a lateralabutted joint, the method comprising the steps of: forming a groovewhich tilts such that a center face of upper and lower groove facessubstantially parallel to each other is lowered with respect to thehorizontal on an initial layer side, and welding by depositing moltenmetal in the narrow groove in layers, with each layer being formed by asingle pass of a welding means, and wherein an angle of inclination of atangent to a bead face at a center point of the bead face in a crosssection of the weld with respect to the horizontal is measured aftertest welding is carried out by using a test piece of the angle of aninclination of zero, so as to select the angle of inclination.